1. Field of the Invention
The present invention relates to power regulators for converting an input voltage into a desired output voltage.
2. Description of Related Art
Power regulators for converting an input voltage into a desired output voltage are used in electronic appliances, OA (office automation) appliances, and the like. These power regulators monitor their output voltage to keep it at the desired level.
Power regulators can be divided roughly into, for example, linear regulators and switching regulators. Linear regulators can be further divided into series regulators and shunt regulators,
FIG. 21 is a block diagram of a conventional power regulator, which will now be described with reference to FIG. 21.
In FIG. 21, an integrated circuit device 1 constituting the power regulator 2000 includes a reference voltage source 2, a control circuit 34, an output stage 5, an input terminal IN, an output terminal OUT, and a feedback terminal FB. The integrated circuit device 1 is configured as, for example, a semiconductor integrated circuit device. The integrated circuit device 1 is provided with, in addition to the input terminal IN, the output terminal OUT, and the feedback terminal FB, other external terminals, which are unillustrated. The control circuit 34 includes a controller 3 and a driver circuit 4.
An output terminal of the reference voltage source 2 is connected to a first input terminal T1 of the controller 3 in the control circuit 34. The reference voltage source 2 generates a reference voltage Vref. The reference voltage source 2 is configured as, for example, a bandgap voltage circuit.
A second input terminal T2 of the controller 3 in the control circuit 34 is connected to the feedback terminal FB of the integrated circuit device 1 across a conductor P1. An output terminal of the controller 3 is connected to an input terminal of the driver circuit 4. The controller 3 compares the reference voltage Vref of the reference voltage source 2 with a feedback voltage fed in via the feedback terminal FB, and outputs a control voltage E1 which is commensurate with the result of the comparison. As the controller 3, for example, an error amplifier configured with an operational amplifier is used.
The driver circuit 4 is used to drive the output stage 5. An output terminal of the driver circuit 4 is connected to a gate G of a MOSFET (metal-oxide-semiconductor field-effect transistor), unillustrated, in the output stage 5. The driver circuit 4 operates based on the control voltage E1 from the controller 3, and outputs a drive voltage E2.
An input terminal of the output stage 5 is connected to the input terminal IN of the integrated circuit device 1. To the input terminal IN, an input voltage Vin is applied. An output terminal of the output stage 5 is connected to the output terminal OUT of the integrated circuit device 1. The output stage 5 is driven based on the drive voltage E2 from the driver circuit 4. The output stage 5 generates an output voltage Vout from the input voltage Vin fed in via the input terminal IN, and feeds the output voltage Vout to the output terminal OUT of the integrated circuit device 1.
The output terminal OUT is connected to a node N2. Between the node N2 and a node N1, a resistor R1 is connected. Between the node N1 and a ground terminal (low-potential terminal) GND, a resistor R2 is connected. The resistors R1 and R2 constitute a voltage division circuit 12. The node N1 is connected to the feedback terminal FB of the integrated circuit device 1. The output voltage Vout is divided by the resistors R1 and R2. Thus, a feedback voltage Vfb appears at the node N1, and the feedback voltage Vfb is fed to the feedback terminal FB.
To the output terminal OUT, a load 9 is connected. The load 9 is, for example, a CPU, MPU, sensor, motor, or the like.
In the conventional power regulator 2000, due to an error in mounting the feedback terminal FB, an error in mounting an externally fitted resistor, or any other inadvertent accident or the like, a disconnection (a broken wire or the like) X can occur between the node N1 and the feedback terminal FB, leaving the feedback terminal FB in an open state. This brings the potential at the feedback terminal FB into an indefinite state. With the potential at the feedback terminal FB in an indefinite state, noise or the like may cause the controller 3 to output an abnormal voltage. This inconveniently leads to the load 9 connected to the output terminal OUT operating in an abnormal condition or deteriorating.
FIG. 22 is a schematic diagram showing relevant potentials observed when the power regulator 2000 is operating normally and when the feedback terminal is open. Now, the circuit operation of the power regulator 2000 will be described with reference to FIGS. 21 and 22.
When the power regulator 2000 is operating normally, the feedback voltage Vfb at the feedback terminal FB is stable. Accordingly, the output voltage Vout at the output terminal OUT also is stable.
On the other hand, when the feedback terminal of the power regulator 2000 is open, the feedback voltage Vfb at the feedback terminal FB is indefinite, and the output voltage Vout at the output terminal OUT also is indefinite.
To solve the above problem, various approaches have been adopted
Japanese Patent Application published as No. 2012-249464 (hereinafter Patent Document 1) discloses a DC-DC converter along with an electronic appliance employing it, wherein a capacitor is fitted between a boot conductor, to which the side of a bootstrap circuit opposite from a switching device is connected, and a feedback conductor. The DC-DC converter disclosed in Patent Document 1 takes little time to start outputting an output voltage lower than the input voltage after the time point that the feedback conductor becomes open. This prevents, even with the feedback conductor open, an excessive voltage from being output.
Japanese Patent registered as No. 3600915 (hereinafter Patent Document 2) discloses a switching power supply device along with an electronic appliance incorporating a display device, wherein the peak of the voltage at the series connection node between a coil and a switch in the switching power supply device is detected to serve as a second detection voltage for overvoltage protection. This helps perform overvoltage protection reliably when a feedback circuit or a component such as or a rectification diode is left in open connection, without a need to provide a switching control IC with an additional terminal for overvoltage protection.
With the DC-DC converter and the electronic appliance employing it disclosed in Patent Document 1, the target of application of the technology involved is limited to switching power supplies that include a boot strap circuit. Thus, the technology cannot be applied to linear regulators.
Likewise, with the switching power supply device and the electronic appliance incorporating a display device disclosed in Patent Document 2, the target of application of the technology involved is limited to switching power supplies, and thus the technology cannot be applied to linear regulators.